1. Field of the Invention
The present invention relates generally to an electronic power steering apparatus for vehicles, and more particularly to improvements in an electric power steering apparatus having a steering device with variable steering ratio.
2. Description of the Related Art
In recent years, electric power steering apparatuses have been used extensively to reduce the manual steering effort or force needed to turn the steering wheel to thereby provide a comfortable steering touch to the driver. The electric power steering apparatuses of the type concerned are constructed such that an assist torque generated by an electric motor in proportion to the steering torque is transmitted to the steering system to reduce the manual steering force required by the driver.
Some known electric power steering apparatuses include a steering device or unit having a steering ratio (the number of 360.degree. turns of the steering wheel that are required to swivel or pivot the front wheels all the way from left to right) which is variable in response to the vehicle velocity. One example of such electric power steering apparatuses is disclosed in Japanese Patent Laid-open Publication No. HEI 7-257406.
The disclosed steering device includes an input shaft connected to a steering wheel and rotatably supported by a support member in eccentric relation to the support member. The support member is rotated by a motor to displace the axis of the input shaft to thereby vary the eccentricity (eccentric distance) between the axis of the input shaft and the axis of an output shaft which transmits the steering force to the front wheels. With this varying eccentricity, the ratio of the rotational angle of the input shaft to the rotational angle of the output shaft, namely, the ratio (steering ratio) of the rotational angle of the steering wheel to the swivel angle of the front wheels is variable. Conventional electric power steering apparatuses in which the steering device is incorporated are generally constructed as follows.
As shown in FIG. 8, the conventional electric power steering apparatus 100 includes a steering wheel 101 operatively connected via a steering shaft 102 and universal joints (not shown) to a rack-and-pinion mechanism (not shown) of a steering device or unit with variable steering ratio (corresponding to the steering device disclosed in the Japanese publication specified above). When the steering wheel 101 is actuated, a steering system produces a steering torque which is detected by a steering torque sensor 104. The steering torque sensor 104 generates a detection signal indicative of the steering torque and sends it to a control means or unit 105 which in turn generates a control signal. Based on the control signal, an electric motor 106 produces an assist torque proportional to the steering torque. The assist torque is applied to a rack of the rack-and-pinion mechanism. The torque sensor 104 is disposed around the steering shaft 102.
The control unit 105 also generates, on the basis of an eccentricity signal indicative of an eccentricity of the input shaft 112 detected by a displacement sensor 111 and a vehicle velocity signal indicative of a velocity of the vehicle detected by a vehicle velocity sensor 113, a steering ratio control signal responsive to the vehicle velocity. Based on the steering ratio signal, a steering ratio control motor 114 drives the steering device 103 to control the steering ratio of the steering device 103.
Because the steering device 103 and the steering torque sensor 104 are structurally independent from one another and provided at different positions spaced remotely from one another, the conventional electric power steering apparatus has various problems, as enumerated below.
(1) The steering torque senor 104 requires a performance test which is performed separately from a performance test of the steering device 103. Such a test process is tedious and time-consuming.
(2) The steering torque sensor 104 requires a separate assembling process additional to the one for assembling the steering device 103 with the vehicle body. The separate assembling process is tedious and time-consuming and also increases the assembling cost. These devices 103, 104 further require separate sets of harnesses which are uneasy to arrange or wire. Especially, because the harness of the steering torque sensor 104 is composed of relatively thin signal-line conductors, care must be taken not to damage or otherwise break any of the conductors during wiring operation. The wiring operation is, therefore, low in efficiency.
(3) The respective components of the steering device 103 and steering torque sensor 104 increase the number and cost of the parts used because they cannot be used in common regardless of the presence of a steering torque sensor provided around the steering shaft 102.
A consideration of the foregoing problem may lead one having ordinary skill to a concluding that the steering torque sensor 104 should preferably be incorporated in the steering device 103. However, a mere combination of these two components will encounter a difficulty because the steering torque sensor should be disposed around an input shaft of the steering device which is adapted to be displaced in a lateral direction.